1. Field of the Invention
The present invention relates to the manufacture of corrugated paper board, and, more specifically, to the so-called corrugator belts which run on the corrugator machines used to manufacture that variety of paper board.
2. Description of the Prior Art
The manufacture of corrugated paper board, or box board, on corrugator machines is well-known in the art. On such machines, corrugator belts pull a web of corrugated board first through a heating zone, where an adhesive used to bond layers of the web together is dried or cured, and then through a cooling zone. Frictional forces between the corrugator belt, specifically the face, or board, side thereof, and the web are primarily responsible for pulling the latter through the machine.
Corrugator belts should be strong and durable, and should have good dimensional stability under the conditions of tension and high temperature encountered on the machine. The belts must also be comparatively flexible in the longitudinal, or machine, direction, while having sufficient rigidity in the cross-machine direction to enable them to be guided around their endless paths. Traditionally, it has also been desirable for the belts to have porosities sufficient to permit vapor to pass freely therethrough, while being sufficiently incompatible with moisture to avoid the adsorption of condensed vapor which might rewet the surfaces of the corrugated product.
As implied in the preceding paragraph, a corrugator belt takes the form of an endless loop when installed on a corrugator machine. In such form, the corrugator belt has a face, or board, side, which is the outside of the endless loop, and a back side, which is the inside of the endless loop. Frictional forces between the back side and the drive rolls of the corrugator machine move the corrugator belt, while frictional forces between the face side and the web of corrugated board pull the web through the machine.
Corrugator belts are generally flat-woven, multi-layered fabrics, each of which is trimmed in the lengthwise and widthwise directions to a length and width appropriate for the corrugator machine on which it is to be installed. The ends of the fabrics are provided with seaming means, so that they may be joined to one another with a lacing cable when the corrugator belt is being installed on a corrugator machine.
In a typical corrugator machine, the heating zone comprises a series of hot plates across which the web of corrugated board is pulled by the corrugator belt. A plurality of weighted rollers within the endless loop formed by the corrugator belt press the corrugator belt toward the hot plates, so that the corrugator belt may pull the web across the hot plates under a selected amount of pressure. The weighted rollers ensure that the web will be firmly pressed against the hot plates, and that frictional forces between the corrugator belt and the web will be sufficiently large to enable the belt to pull the web.
In a new generation of corrugator machines, the weighted rollers have been replaced with air bearings, which direct a high-velocity flow of air against the back side of the corrugator belt and toward the hot plates to force the corrugator belt toward the hot plates. In order to prevent the high-velocity air flow from passing through the corrugator belt, which would cause the belt to lift from the web of corrugated board and allow the belt to slip in the running direction relative to the web, leading to poor contact between the web and the hot plates and ultimately to poor, non-uniform bonding in the laminated corrugated board product, the back sides of the corrugator belts used on machines having air bearings have a layer of polymeric resin material, which is impermeable and seals the corrugator belt to prevent air from passing therethrough.
In an even newer generation of corrugator machines, the corrugator belt which presses the web of corrugated board against the hot plates has been eliminated to avoid such belt-related problems as seam mark, edge crush, edge bonding and warping. Instead, a pair of belts downstream from the heating zone in a cooling zone sandwich the web of corrugated board from above and below and pull it through the heating zone.
It has been found that the corrugator belts currently available have not worked satisfactorily when installed on this latest generation of corrugator machines. A present, corrugator belts have a needled or woven surface with a coefficient of friction, relative to corrugated board, in a range from 0.15 to 0.20. As the corrugator belts contact the web of corrugated board only in the cooling zone over a total area much less than that characterizing older machines, current belts have not been able to generate frictional forces large enough to pull the web through the corrugator machine.
Clearly, corrugator machines of this most recent type require corrugator belts whose surfaces have a greater coefficient of friction, relative to corrugated board, than those currently available, so that they will be able to generate the required frictional forces. This need is met by the present invention.